Topic: Keihin CR Carburretor Behavoir - Tuning or Fact?

[RRRToolSolutions]

I could use some input from those of you running the CR's, please. Mine are newly installed on my 836 bike. I've done a lot of carburretor tuning over the years, but this set-up has me puzzled. They are fresh out of the box and have the K&N's installed that are sold with them. I started it up and rode it Saturday for the first time since the rods, studs, and chain upgrades. I only rode it 10 miles as rain was going to happen - just wasnt sure when. Yesterday I rode some more to check for leaks and bring it up to temps, then to check for idle adjustments at the air screw and idle speed. - rode a little better.

Here is the deal -

These only have about a 1/3 throttle turn from fully closed to fully opened, so metering the throttle is quite a bit more delicate than stock. The carbs will not allow any "quick" throttle opening. If you twist the throttle open at low, middle, or anywhere before 7,000 - the huge air signal is dry and therefor causes the motor to die momentarily. If you twist it open at 7,000, you get a few blubbers (rich), but it quickly goes into it's hardest and most exciting pull of it's range. If these transitions in throttle are made moderately - then it never has a dead spot, lean spit, or rich blubbering. This makes me thing the jetting is ok. I also measured cylinder head temps throughout the ride to confirm I wasn't melting something. Normally a problem like this - I'd raise the needle clip to cure what would be a dead spot in the middle and a lean spit, then drop the mains by 1 size to cure a rich condition that persisted regardless of application. Here I can get low, middle, high perfomnace from each circuit as -is, but you can't open the throttle before the engine can use it. On a regular carb - I'd never be able to transition through it.

I would say an acellerator pump is designed to cure such problems on autos. They squirt about 5cc of raw gas into the intake manifold when the pedal is floored to help cure the exact symptom.  Am I missing something about tuniing the CR's or am I finding something that is to be lived with and expected? or is there a cure?

Thanks,
Gordon

[Jerry Rxman Griffin aka MuthaF'er]

Gordon,

I'm not sure about the CRs yet and I'm sure I'll learn from your trials,

BUT, I gotta say I really like your 2007 CB750K9 Honda! 

[Jim F]

I am wondering if the motor just cant keep up with what the carbs are doing  with a 1/4 turn or  a 1/3 turn type throttle
not sure if that makes sense but

My 836 has CR,s but my throttle is a 3/4 turn and it takes longer
to go from closed postion to wide open

just a thought

Have also talked to some people about these and they say they are very hard to get jetted just right
they are really nice once you get them there

again......just a thought

Jim

[Jim F]

something that I haven't done is put it on a dyno with a sniffer to see what the power is doing

So I cant assume that my jetting is right either

My mains are 120s

I knew the poilots are rich at 58s.......blubbers like hell on the slow end but I don't need the chock to start it.............

240 air jets

Y-8 Needles

clip is in the middle position

call me tomorrow, I sent the e-mail with my cell number

Jim

[RRRToolSolutions]

Jim, thanks for the confirmation of jetting. With the exception of the 230 air-jet, yours has all of the jetting of a 33mm. I would think they all use the same carb, just a larger bore in the venturi - the outside dimensions would be the same. I know Mike's are 31's and from the outside look identical.

By the way, mine are 120 mains, 60 pilots, 250 air jet, Y8 needle in the 4th position, air screw likes 1/2 turn out - no more else it lean spits.


I called and spoke with Sudco. They gave me some information - Some I didn't want to hear 1) They haven't yet written any instructions for tuning nor put together a tuning guide for the Keihin carburettors. 2) My result is about right for a performance carb. It can't react suddenly to change and they have developed the FCR's with a fuel pump to address this problem. I should replace my throttle or transition slower when going from part to full throttle as signal will be lost.

I re-read my Sudco Mikuni tuning manual this weekend and I'm not going to follow it's instruction either. It states to run the main jet giving the highest top speed in top gear - then go up 1 size for safety. I don't have the nads to go full-out...

[sparty]

ILbikes,

I am guessing here, but the problems you are experiencing may be caused by simple misconfigurations.

1. From your photo, it looks like your CR carbs are aligned at a slight downward angle. I believe it is necessary to have your carbs as close to perfectly level as possible otherwise, your float level will not be correct.

2. Your throttle cables must be the incorrect size.  You should have much more than 1/3 of turn on your throttle.  Your CRs are problematic because they are not fully closing or opening.

3. I noticed that you have the Dyna 2000 ignition.  Be sure that you have the timing correct and the advance curve set correctly.

Sparty

[RRRToolSolutions]

Art, actually the venturis are in perfect alignment with the intake boots and manifolds and are within the tolerance for lean angle. I've got huge Mikunis on my H2's that are much more extreme. As the floats are round, they will act upon the float valve at the same time as if level. I do lose some closing pressure since the lift is not entirely vertical, but still plenty of pressure to seal the fuel valve.

The timing is spot-on at 36 degrees full advance for 1/4 and 2/3, so both pick-ups are perfect. The rev limiter is set on 11,000 and the curve on # 4 per spec. for two valve applications and normal compression. As for throttle, I'm using a OEM grip, case, and cable system for the K4. Some mods were necessary for the cable installation, but I am getting full closure and full opening and the proper open/close tension. I checked those performances before ever adjusting the push/pull cables or installing the filters. These just don't transition quickly and I'm surprised by that gap I've got when these are quickl opened.

Thanks for the input. I don't mean to sound ungrateful - it's just that I've been around the block a time or two... These CR's are not the Mikunis I'm used to dealing with.

Regards,
Gordon

[sparty]

Thanks for the input. I don't mean to sound ungrateful - it's just that I've been around the block a time or two... These CR's are not the Mikunis I'm used to dealing with.

Regards,
Gordon

Gordon,

No problem, but in my experience, 1/3 throttle twist with the CRs is not the norm.

Art

[Jim F]

Gordon

Call a company called motorcyclecarbs.com,Inc

you need to talk to Robert

He has helped me allot with my carbs in the past and can also get hard to find parts

Here is the web site for you and all.............http://www.motorcyclecarbs.com/index.cfm

Jim

[Jinxracing]

Gordon, I know next to nothing about carb tuning, but from a mechanical point of view would it be possible to attach the end of your throttle cable further out from the fulcrum of the carb linkage rotational center? Essentially giving you a longer "lever" and more (and therefore slower) throttle travel?

[Sam Green Racing]

Gordon, I have not been able to contribute as all our settings were done by our dyno man three years ago.

The only thing I can add, the settings as they are now for the 736 motor are miles out with the 836 power plant.

I think Chris said the mains are 121 and it is miles to rich. After 9 passes dow the strip, the plugs started to soot up.

Sam.

[andy750]

Gordon, if you havent sought advice from Axl (Satanic Mechanic) yet you should ask him or at his Forum:

http://www.satanicmechanic.de/board/viewforum.php?f=6

good luck,
Andy